A word used instead of saying:"fooled you!" after a prank. It first appeared in the series of "The Big Bang Theory" and is used by the character named Sheldon. It comes from the word "zing" which means to fool or rediculous someone. and just for a cool slang, the added the "ba" to it. And later the series added the "a" to it, and then it became "Bazinga".

So this is a conversation between all the characters where Sheldon fools everybody with his classic joke and then say:"Bazinga!" instead of fooled you!" or "Gotcha!":
Howard and Rajesh: "We're going to the goth club!"
Sheldon:"I've always wanted to go to a goth club!...... Bazinga!"